Heat Transfer Enhancement Caused by Sliding Bubbles

Abstract

Measurements that illustrate the enhancement of heat transfer caused by a bubble sliding under an inclined surface are reported. The data were obtained on an electrically heated thin-foil surface that was exposed on its lower side to FC-87 and displayed the output of a liquid crystal coating on the upper (dry) side. A sequence of digital images was obtained from two cameras: one that recorded the response of the liquid crystal and one that recorded images of the bubble as it moved along the heated surface. With this information, the thermal imprint of the bubble was correlated to its motion and position. A bubble generator that produced FC-87 bubbles of repeatable and controllable size was also developed for this study. The results show that both the microlayer under a sliding bubble and the wake behind the bubble contribute substantially to the local heat transfer rate from the surface. The dynamic behavior of the bubbles corresponded well with studies of the motion of adiabatic bubbles under inclined plates, even though the bubbles in the present study grew rapidly because of heat transfer from the wall and the surrounding superheated liquid. Three regimes of bubble motion were observed: spherical, ellipsoidal and bubble-cap. The regimes depend upon bubble size and velocity. A model of the heat transfer within the microlayer was used to infer the microlayer thickness. Preliminary results indicate a microlayer thickness of 40–50 μm for bubbles in FC-87 and a plate inclination of 12 deg.

Local heat transfer coefficients in spray cooling systems measured with temperature oscillation IR thermography

“…By measuring the color or the hue of the TLC's, an estimate of their temperature can be calculated. The application of this measurement technique for mapping heat transfer coefficients is well described by Stasiek [8] and Bayazit et al [9]. Dano et al [10] investigated the local heat transfer coefficients under an array of air jets with crossflow on an area of 49 cm 2 using a CCD camera with 640 Â 480 pixel resolution and a transparent orifice plate to view the impingement surface.…”

Blasenaufstieg an geneigter Wand in Wasser und Flüssigmetall mit Magnetfeld

“…Untersuchungen zu Blasen, die in transparenten Flüssigkeiten an geneigten Begrenzungen aufsteigen, sind mit eingebrachten Blasen [1 -4] sowie mit durch Verdampfung an beheizter Wand entstehenden Blasen bekannt [5]. Eine berechnete Übersicht zur Blasendeformation gibt Norman [6] als Diagramm Bond-bzw.…”

Visualization of the wake behind a sliding bubble

“…For inclined surfaces, bubbles tend to impact, bounce, and subsequently slide under the surface. To date, both vapor and gas bubbles have been observed to enhance convective heat transfer rates as they slide [6,[26][27][28]. A significant portion of this enhancement stems from the turbulent mixing of the surrounding fluid, which is induced by the unsteady wake [3].…”